Fixed and step attenuators are used in a wide variety of applications for signal conditioning and level control. Specifically, attenuators may reduce signal levels, match impedances of sources and loads, and measure gain or loss of a two-port device. Step attenuators are widely used in electronic systems to control the amplitude of signals. Step attenuators, as opposed to fixed value attenuators, have attenuation values that may be selected by electronically or digitally controlled lines, and there may be one attenuation state or multiple attenuation states. They are usually made with resistors having fixed resistances that are connected by electrically controllable switches. The switches may be mechanical (e.g., microelectromechanical systems (“MEMS”) switches or traditional relays) or made with semiconductors (e.g., Field Effect Transistors (“FETs”)). These resistors are usually connected to form “Pi” (see FIG. 1A) or “T” attenuators (see FIG. 1B), but it appreciated by those skilled in the art that other topologies are possible.
Generally, there are a number of characteristics of step attenuators that are important for time domain signal control at high frequencies:
(a) The attenuator should be matched to the transmission or circuit characteristic impedance, often called Zo, which is typically about 50 or 75 ohms;
(b) The insertion loss of the step attenuator should be as small as possible to avoid signal loss;
(c) The attenuation step size, which is the difference in dB between the maximum attenuation and minimum attenuation for each attenuation level, should be constant with frequency; and
(d) The group delay should be constant over the frequency of operation to ensure time domain response fidelity.
Typically, an attenuator match in an electronic system is established by the impedance values and the resistance of the switches in the attenuator in their “ON” state. It is appreciated that while the attenuator may have a perfect match with the nominal values of the impedances and switch “on resistance,” this match will change whenever these impedances vary within their manufacturing tolerances. Generally, switch devices (such as PIN diodes and FETs) are modeled simply as impedances in the “ON” state, and capacitors in the “OFF” state. As an example, an ideal switch (FIG. 1C) has zero impedance in the “ON” state and infinite impedance and zero capacitance in the “OFF” state. Generally, for a switch, there is an ON resistance “Ron” (FIG. 1C) and an OFF capacitance “Coff” (FIG. 1C) and it is appreciated that there will be a manufacturing tolerance for Ron that will influence the attenuator match.
In an integrated circuit (“IC” or “chip”), impedances are typically realized with lightly doped semiconductor regions or traces of resistive metals. Therefore, in an IC type of switch, there is a manufacturing tolerance for the impedances and the resulting variation in the impedances values and the Ron of the IC switch generally limits the accuracy of the attenuator match. In an example where the impedance is a resistor, the resistor accuracy in an IC may be +/−15% and the Ron accuracy may be +/−5%.
Unlike monolithic attenuators on an IC, a known method to improve the match tolerance for attenuators made with discrete parts is to select impedances and switches with tighter tolerances. Unfortunately, this adds to the cost of the attenuator. If the attenuator is implemented monolithically in an integrated circuit, the resistors may be trimmed on chip (e.g., by heat or with a laser). As an alternative, selected or trimmed discrete resistors may be added externally to the chip. Unfortunately, both these approaches add cost and complexity to the attenuator assembly.
The attenuation step value typically varies with frequency due to electrical parasitics and the inherent limitations of the switching elements. As an example, in an attenuator with series switch elements, the capacitance across the series switches while in the “OFF” state generally causes the attenuation step to decrease with increasing frequency. This may be compensated for by adding low pass filters to the attenuator. These low pass filters are usually implemented with fixed or switchable shunt capacitors. Unfortunately, while these shunt-capacitor low pass filters are effective in extending the bandwidth over which the attenuation is constant, they usually increase the minimum attenuation and add complexity to the design and implementation of the attenuator assembly.
Therefore, there is a need to improve the accuracy and tolerance of the attenuator match in a way that is less complex and expensive than present systems, and also to individually adjust the impedance match for each attenuation state so as to improve accuracy and correct for impedance drifts over time. Additionally, there is a need to improve the accuracy of the attenuation step at higher frequencies with small degradation to the minimum attenuation, as well as improve group delay flatness at higher frequencies, which improves time domain response fidelity.